Magnesium alloys of the rare earth metals: systematics and properties

Recent literature data on the possible technological beneficial effects of the addition of rare earth metals (Rj to magnesium based alloys are briefly reported. The constitutional properties (crystallochemistry; thermodynamics, phase equilibriaj of binary R-Mg alloys are reviewed, subdividing the different groups of systems as the following: those given by the light R, those formed by the heavy R (to which yttrium has to be added) and by scandium and finally the systems with the "divalent" rare earths, europium and ytterbium which show a number of analogies with the typical divalent alkaline earth metals). Properties and characteristics of the Mg-rich alloys are especially described and discussed (Mg solid solutions, stoichiometries and structures, stable and metastable equilibria corresponding to the Mg-richest phases). Several properties of R-Mg alloys change smoothly as a function of the atomic dimension of the trivalent rare earth: similar dependence is also observed in ternary R'-R"-Mg alloys, formed by two different trivalent rare earth metals, for which the averaged radius of the R'-R" mixture involved may be used as the corresponding reference parameter. This allows a semi­quantitative check of the reliability of a given R-Mg system and can be used as a first criterion for predicting phase stoichiometries, crystallochemistry and properties of selected ternary alloys in order to design and plan their structures and characteristics.